Microcontroller units (MCU) provide a processor based mixed-signal application integrated circuit that is typically referred to as a system on a chip (SOC). Typically, a processor is provided of the 8051 type, a conventional processor, with memory such as Flash. Data converters are provided for interfacing with the analog domain so as to allow analog data to be collected from various types of sensors. There are also provided various oscillators and the such on the integrated circuit. These SOC integrated circuits are typically directed toward instrumentation type applications that requires interface with sensors and the such, and which can provide some processing of the information from these sensors in a digital domain.
In some applications of MCU based SOCs, there is a requirement for a low power mode of operation. For example, when these devices are associated with implanted apparatus, such as knee joints, there is a desire that the battery last as long as possible. Although there are techniques for charging batteries external to the human body, it is still desirable to minimize the size of the battery if possible. Thus, these SOCs have what is termed a “sleep mode” associated therewith. The sleep mode allows the SOC to be placed in a very low power mode of operation wherein substantially all operations are suspended or halted to conserve power.
There are a number of ways to implement the sleep mode of operation. The first is a complete power down of the chip wherein the configuration information and the such can be stored in the on-chip memory prior to power down. This is the lowest power mode of operation, since there is virtually no current drawn when the chip is powered down. However, this mode of operation is required to store all state information of the CPU and, when it is again powered up, it must reload the stored information such that the logic states at power down can be recovered. A second mode that is implemented is that associated with suspending the clock. When the clock is suspended, the digital portion of the integrated circuit is maintained at the last known state for all transistors. Therefore, when one inverter circuit has a “1” on the input thereof, the output thereof will have a “0” on the output and this will remain fixed until the sleep mode is terminated. This reduces power, as nodes are not being toggled between logic states, which draw current. However, in this mode of operation, there is still some inherent leakage associated with the transistors. For example, even though an N-channel transistor is turned off due to the fact that the gate voltage is disposed at a voltage equal to or lower than the source voltage, there is still the possibility for some “sub-threshold conduction.” This is the condition that, even though the transistor is operating below its threshold voltage, the transistor can still conduct. As the dimensions of the transistors are reduced with advancing technology, this leakage current increases. Thus, even though the clock is suspended, there is still the possibility of a not insignificant current draw occurring due to the leakage through the transistors that are turned off.